A great variety of techniques are being used for the synthesis of nanoparticles of inorganic compounds. Most of these techniques suffer from lack of precision in controlling particle size and properties. The current state of the art in the synthesis of semiconductor nanocrystals involves the use of high temperature batch reactors. This process uses a hot coordinating solvent, such as hexadecylamine and trioctyl-phosphine, in which the reactants are injected with a syringe. Particles grow as a function of time and samples are taken at specific times to obtain populations of a certain average size. It is difficult to precisely control particle size distribution in such reactors and very difficult to isolate particles with a specific, pre-determined particle size. Using this approach, post-processing and functionalization requires many additional steps that can compromise the quality of the particles. Further, the technique cannot be scaled-up easily for industrial production.
Semiconductor nanocrystals are currently under intense investigation due to their unique, size-dependent optical and electronic properties that differ significantly from those observed in the bulk material (Rossetti et al., “Quantum Size Effects in the Redox Potentials, Resonance Ramam Spectra, and Electronic CdS Crystallites in Aqueous Solution,” J. Chem. Phys. 79(2):1086-1088 (1983); Fendler, J. H., “Self-Assembled Nanostructured Materials,” Chem. Mater. 8(8):1616-1624 (1996); Murray et al., “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies,” Annu. Rev. Mater. Sci. 30:545-610 (2000); Alivisatos, A. P., “Semiconductor Clusters, Nanocrystals, and Quantum Dots,” Science 271:933-937 (1996)). When at least one dimension of the nanocrystals becomes smaller than the corresponding de Broglie wavelength or Bohr radius (mean separation of an optically excited electron-hole pair), quantum confinement phenomena take place that can change the nanocrystal properties (Fendler, J. H., “Self-Assembled Nanostructured Materials,” Chem. Mater. 8(8):1616-1624 (1996)). Nanocrystals confining electron-hole pairs in zero dimensions (quantum dots) can exhibit size-dependent luminescence, broad excitation by any wavelength smaller than the emission wavelength, high brightness, high sensitivity, and high photostability (Murray et al., “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies,” Annu. Rev. Mater. Sci. 30:545-610 (2000)). Such properties make them useful for a variety of applications, including light emitting diodes, photodetectors and photovoltaics (Alivisatos, A. P., “Semiconductor Clusters, Nanocrystals, and Quantum Dots,” Science 271:933-937 (1996)), and as fluorescent biological labels (Michalet et al., “Properties of Fluorescent Semiconductor Nanocrystals and Their Application to Biological Labeling,” Single Mol. 2(4):261-276 (2001)). On the other hand, semiconductor nanowires and nanorods that confine electron-hole pairs in one dimension have been attracting attention for fundamental studies in mesoscopic physics and for their potential applications as interconnects and functional units in nanoelectronics (xia et al., “One-Dimensional Nanostructures: Synthesis, Characterization, and Applications,” Adv. Mater. 15(5):353-389 (2003)).
The most common synthesis route for II-VI nanocrystals involves reactions between organometallic compounds in a trioctylphosphine (“TOP”)/trioctylphosphine oxide (“TOPO”) and/or hexadecylamine (“HAD”) coordination solvent carried out in small batch reactors operating at ˜300° C. CdSe and CdS quantum dots have been the most common materials grown by this technique (Murray et al., “Synthesis and Characterization of Nearly Monodisperse CdE (E=sulfur, selenium, tellurium) Semiconductor Nanocrystallites,” J. Am. Chem. Soc. 115(19):8706-8715 (1993)). Luminescent ZnSe nanocrystals exhibiting high quantum yield (Hines et al., “Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals,” J. Phys. Chem. B. 102(10):3655-3657 (1998); Revaprasadu et al., “Single-Source Molecular Precursors for the Deposition of Zinc Selenide Quantum Dots,” J. Mater. Chem. 8:1885-1888 (1998)) and (Zn,Mn)Se diluted magnetic nanocrystals (Norris et al., “High-Quality Manganese-Doped ZnSe Nanocrystals,” Nano Lett. 1(1):3-7 (2001)) have also been grown. To grow monodisperse nanocrystal populations the requirements include instantaneous injection and mixing of the reactants, uniform nucleation over the entire mass of the solvent, and perfect mixing during the entire process. Such conditions are difficult to achieve and selective precipitation techniques are used after synthesis to narrow down the size distribution of the nanocrystals (Murray et al., “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies,” Annu. Rev. Mater. Sci. 30:545-610 (2000)). Other reported techniques for growing ZnSe nanocrystals include arrested precipitation (Chestnoy et al., “Higher Excited Electronic States in Clusters of ZnSe, CdSe, and ZnS: Spin-Orbit, Vibronic, and Relaxation Phenomena,” J. Chem. Phys. 85(4):2237-2242 (1986)), sol-gel processing (Li et al., “Preparation and Optical Properties of Sol-Gel Derived ZnSe Crystallites Doped in Glass Films,” J. Appl. Phys. 75(8):4276-4278 (1994)), sono-chemical processing (Zhu et al., “General Sonochemical Method for the Preparation of Nanophasic Selenides: Synthesis of ZnSe Nanoparticles,” Chem. Mater. 12(1):73-78 (2000)), growth in reverse micelles (Quinlan et al., “Reverse Micelle Synthesis and Characterization of ZnSe Nanoparticles,” Langmuir 16(8):4049-4051 (2000)), and vapor-phase synthesis (Sarigiannis et al., “Characterization of Vapor-Phase-Grown ZnSe Nanoparticles,” Appl. Phys. Lett. 80(21):4024-4026 (2002)).
The use of a template is typically required for growing monodisperse particle populations. Microemulsion templates have been employed for such a task. Control of particle microstructure has been achieved by colloidal crystallization in aqueous droplets suspended on the surface of a fluorinated oil (Velev et al., “A Class of Microstructured Particles Through Colloidal Crystallization,” Science 287:2240-2243 (2000)). Monodisperse populations of Si quantum dots, with surfaces passivated by an organic monolayer, were grown by thermally degrading diphenysilane in supercritical octanol (Holmes et al., “Highly Luminescent Silicon Nanocrystals with Discrete Optical Transitions,” J. Am. Chem. Soc. 123(16):3743-3748 (2001)). ZnSe nanocrystals were grown in bis-2-ethylhexylsulphosuccinate sodium salt (AOT) reverse micelles by reacting zinc perchlorate hexahydrate and sodium selenide (Quinlan et al., “Reverse Micelle Synthesis and Characterization of ZnSe Nanoparticles,” Langmuir 16(8):4049-4051 (2000)). Under ideal conditions, reverse micelles could function as identical nanoreactors, thus providing a template for precise control of particle size. In practice, the fast dynamics of droplet coalescence in water-in-oil microemulsions lead to the formation of droplet clusters and polydisperse particle populations (Zhao et al., “Preparation of CdS Nanoparticles in Salt-Induced Block Copolymer Micelles,” Langmuir 17(26):8428-8433 (2001)).
Highly luminescent ZnSe quantum dots have been grown from diethylzinc and Se powder in a coordinating solvent of tri-η-octylphosphine (TOP) and hexadecylamine (HDA) at 270° C. (Hines et al., “Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals,” J. Phys. Chem. B 102(19):3655 (1998)). A single-source precursor has been used to grow ZnSe quantum dots in tri-η-octylphosphine oxide (TOPO) at 250° C. (Revaprasadu et al., “Single-Source Molecular Precursors for the Deposition of Zinc Selenide Quantum Dots,” J. Mater. Chem. 8(8):1885-1888 (1998)). Polymer capping agents have been used to encapsulate and stabilize ZnSe quantum dots grown from zinc chloride and sodium selenosulfate solutions at room temperature (Leppert et al., “Structural and Optical Characteristics of ZnSe Nanocrystals Synthesized in the Presence of a Polymer Capping Agent,” Mater. Sci. Eng. B 52(1):89-92 (1998); Kumbhojkar et al., “Quantum Confinement Effects in Chemically Grown, Stable ZnSe Nanoclusters,” Nanostruct. Mater. 10(2):117-129 (1998)). The as-grown quantum dot populations from the above techniques have a relatively wide size distribution that can be narrowed down by several post-processing steps including size-selective precipitation (Hines et al., “Bright UV-Blue Luminescent Colloidal ZnSe Nanocrystals,” J. Phys. Chem. B 102(19):3655 (1998)). In an effort to grow ZnSe quantum dots and obtain narrow size distributions by use of a template, reverse micelles have been employed (Quinlan et al., “Reverse Micelle Synthesis and Characterization of ZnSe Nanoparticles,” Langmuir 16(8):4049-4051 (2000)), but the high rate of micelle-micelle coalescence (Alexandridis et al., “Thermodynamics of Droplet Clustering in Percolating AOT Water-in-Oil Microemulsions,” J. Phys. Chem. 99(20):8222-8232 (1995)) prevented a narrow focusing of the size distribution. Recently, one group has reported the growth of luminescent ZnSe quantum dots using a new microemulsion template that has slow droplet-droplet coalescence kinetics and thus allows the narrow focusing of the particle size distribution of the as-grown quantum dots by forming a single quantum dot per droplet (Karanikolos et al., “Synthesis and Size Control of Luminescent ZnSe Nanocrystals by a Microemulsion-Gas Contacting Technique,” Langmuir 20(3):550-553 (2004)).
Preparation of ZnSe nanowires and nanorods with diameters of several tens of nanometres has been reported in the literature by a variety of techniques, including metal organic chemical vapour deposition (MOCVD) using colloidal Ag particles as catalyst (Zhang et al., “Growth and Luminescence of Zinc-Blende-Structured ZnSe Nanowires by Metal-Organic Vapor Deposition,” Appl. Phys. Lett. 8(26)3:5533-5535 (2003)), solvothermal processing (Wang et al., “Synthesis and Characterization of MSe (M=Zn, Cd) Nanorods by a New Solvothermal Method,” Inorg. Chem. Commun. 2(3):83-85 (1999)), electrodeposition inside a porous alumina film (Kouklin et al., “Giant Photoresistivity and Optically Controlled Switching in Self-Assembled Nanowires,” Appl. Phys. Lett. 79(26):4423-4425 (2001)), laser ablation using Au as catalyst (Jiang et al., “Zinc Selenide Nanoribbons and Nanowires,” J. Phys. Chem. B 108(9):2784-2787 (2004)), a self-catalysed vapour-liquid-solid (VLS) method (Zhu et al., “Preparation and Photoluminescence of Single-Crystal Zinc Selenide Nanowires,” Chem. Phys. Lett. 377(3-4):367-370 (2003)), and a surfactant template method (Lv et al., “Growth and Characterization of Single-Crystal ZnSe Nanorods via Surfactant Soft-Template Method,” Solid State Commun. 130(3-4):241-245 (2004)).
The control of the shape and structure of nanoscale materials, like nanowires, focuses mostly on carbon nanotubes that are synthesized through a catalytic process employing metal (gold) nanoparticles as “seeds”. This technique has also been used for demonstration of the synthesis of semiconductor nanowires. However, a critical issue is the poor control over nanowire size and lack of an ability to grow more complex shapes, such as bi-continuous structures, honeycombs, and the like.
Thus, there is a need for developing techniques that enable precise control of the shape, size and orientation of these materials. A variety of techniques have been reported that utilize templates or surfactant-mediated growth for controlling the size and shape of meso- and nano-scale materials. Examples include the use of amphiphilic compounds as a template for the growth of zirconium oxide with a mesostructured framework (Wong et al., “Amphiphilic Templating of Mesostructured Zirconium Oxide,” Chem. Mater. 10(8):2067-2077 (1998)), the surfactant-mediated growth of monodisperse iron oxide nanoparticles (Teng et al., “Effects of Surfactants and Synthetic Conditions on the Sizes and Self-Assembly of Monodisperse Iron Oxide Nanoparticles,” J. Mater. Chem. 14(4):774-779 (2004)), the growth of colloidal crystals in aqueous droplets suspended in a fluorinated oil (Velev et al., “A Class of Microstructured Particles Through Colloidal Crystallization,” Science 287:2240-2243 (2000)), and the synthesis of Si quantum dots by thermally degrading diphenylsilane in supercritical octanol (Holmes et al., “Highly Luminescent Silicon Nanocrystals with Discrete Optical Transitions,” J. Am. Chem. Soc. 123(16):3743-3748 (2001)).
The present invention overcomes deficiencies in the art by employing novel templates based on amphiphilic systems that enable precise control of size, structure, shape and composition of nanostructured materials.